Why Excess Cyanide Can Be Detrimental to Pd-Catalyzed Cyanation of Haloarenes. Facile Formation and Characterization of [Pd(CN)₃(H)]^2- and [Pd(CN)₃(Ph)]^2-

Unexpected side reactions at the Pd center have been identified for Pd-catalyzed cyanation of haloarenes. In the presence of excess cyanide, these reactions can proceed with ease, interfering with every key step of the catalytic cycle and thereby disrupting the process. Mixed PPh3/CN- Pd(0) species easily activate the C−N bond of Bu4N+ to produce cleanly and selectively a novel hydride [(CN)3PdH]2- which is stable in the presence of extra CN- but easily undergoes HCN reductive elimination under CN--free conditions. While slow addition of 1 equiv of CN- to [(Ph3P)2Pd(Ph)I] leads to PhCN formation, quick mixing of the same complex with a 4-fold excess of CN- does not produce PhCN but rather leads to [(CN)3Pd(Ph)]2-. Reductive elimination of PhCN from the latter is strongly inhibited by small quantities of free cyanide.